As described in Patent Document 1, some existing search and tracking support apparatuses estimate a two-dimensional moving velocity of a detection object, which is a tracking target, using image information obtained from a radar apparatus, and detect a time when the tracking target detection object is at its point of closest approach and the position of the object at that time from the two-dimensional moving velocity of the tracking target detection object and the two-dimensional moving velocity of the apparatus itself.
Such apparatuses can compute the distance of closest approach of the detection object using the two-dimensional moving velocity of the detection object and the two-dimensional moving velocity thereof (the distance of closest approach=“0” in these existing apparatuses).
In addition, existing radar apparatuses for a motor vehicle scan a beam in an azimuth direction and consider the azimuth direction in which the intensity of reflection from the target is maximized as the azimuth direction of the target. However, in this method, even when the target is located in the upper front direction, and therefore, the target and the vehicle can pass each other, the radar apparatus determines that the target is an obstacle to the vehicle. Thus, when going under an overpass or a traffic sign, the vehicle may receive an incorrect warning or an incorrect brake signal.
On the other hand, Patent Documents 2 to 6 describe radar apparatuses that can determine whether a motor vehicle and a target can pass each other by acquiring the height information about the target.
Patent Document 2 describes a method for determining the height of a target that is stationary by determining whether the effect of multipath appears.
Patent Document 3 describes a configuration in which a beam is scanned in the horizontal direction by moving an antenna mechanically, and the beam is scanned in the vertical direction by using phased array antenna.
Patent Document 4 describes a configuration in which a traveling wave antenna that changes the irradiation direction in accordance with the frequency is used, and the elevation angle at which an object is located is determined using the intensity of a pulse while changing the frequency first. Subsequently, FM-CW modulation is performed mainly for the frequency corresponding to the elevation angle. By switching an electronic switch and performing horizontal scanning, detection can be performed effectively even for a sloping road.
Patent Document 5 describes a configuration in which a traveling wave antenna is employed, and a beam is scanned in a vertical direction using frequency switching or using a phase shifter.
Patent Document 6 describes a configuration in which detection is made by performing electronic scanning in a horizontal direction and performing scanning in a vertical direction using a phase mono-pulse method.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-330664
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-153946
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2003-207559
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2004-101347
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2004-226158
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 11-287857
The apparatus described in Patent Document 1 can detect in which azimuth direction a detection object is located and in which direction the object is moving with respect to the apparatus, thus highly accurately measuring the distance to the object. However, it is difficult to detect the direction (the azimuth direction) highly accurately. Therefore, the accuracy of azimuth direction detection is decreased. As described above, since the two-dimensional moving velocity is computed using the computed low-accuracy azimuth direction, the estimated computed distance of closest approach includes an error to some extent. If, as described in Patent Document 1, tracking is performed using an image output from a radar apparatus, such an error is acceptable. However, for a radar apparatus mounted in a motor vehicle that detects another vehicle coming from the front direction, such an error is not acceptable in order to prevent collision.
In addition, in order to highly accurately compute the distance of closest approach, the apparatus described in Patent Document 1 needs a large number of sampling points, and therefore, the computing time of the distance of closest approach is increased. However, for a radar apparatus mounted in a motor vehicle, the relative velocity between the vehicle and the oncoming vehicle is high. Accordingly, in this case, the detection and the subsequent collision avoidance operation are performed over a short distance compared with the case of, for example, a boat. Consequently, the distance of closest approach needs to be computed in a short time.
On the other hand, in Patent Document 2, the height is obtained using a distance at which the effect of multipath appears. However, in practice, it is difficult to detect the height of a target in a range in which the reception level is very small, since factors other than the multipath that varies the detection output of the radar apparatus, such as vibration of the traveling vehicle, are present.
In addition, in Patent Documents 3 to 6, the height of a target is obtained by scanning a beam not only in a horizontal direction (an azimuth direction) but also in a vertical direction (an elevation angle direction). Accordingly, the beam scanning operation takes time, and the processing system becomes complicated. Thus, the entire apparatus disadvantageously becomes large-scaled.